The invention relates to disc drives. In particular, the invention relates to spindle motors for rotatably driving memory storage discs.
Magnetic disc drives act as mass storage devices for computers by selectively polarizing portions of a magnetic disc surface. In the continual effort to increase storage capacity of magnetic disc drives, the density of data stored on the disc has been continually increased by decreasing the size of the polarized portions of the disc surface. Reduction in the size of the magnetically polarized bit positions on the disc results in a decrease in the signal strength induced in the read/write head as the surface of the magnetic disc passes under it. Accurate data transfer to and from the disc requires minimizing extraneous signals or electronic "noise".
The magnetic disc typically rotates at speeds of up to 7200 rpm and is supported by a spindle rotor hub. Both are driven by a spindle drive motor. The spindle rotor hub is rotatably supported above a base of the spindle motor by ball bearings. Typically, the ball bearings in prior art spindle motors are made of steel, usually 52100 chrome steel.
Several key performance parameters including non-repeatable runout, acoustics, shock resistance, running friction and contamination are effected by properties of the ball bearings. Bearing geometry effects the rotation of the rotor hub and ultimately the rotation of the magnetic disc. With prior an spindle motors, rotation of the magnetic disc has been oval shaped rather than circular. This disuniformity, otherwise known as runout, has been tolerable as long as the runout repeats in a consistent pattern. However, due to waves in the bearing race and imperfect roundness of the bearing balls, inconsistent runout or nonrepeatable runout has become a problem. Inconsistent runout or nonrepeatable runout misaligns data tracks in relationship to the location of the read/write head. Consequently, nonrepeatable runout limits storage capacity of discs by limiting the number of tracks per inch on the discs.
The surface finish, the axial stiffness, and the radial stiffness of ball bearings also has an effect on the vibration of the spindle motor during rotation of the memory storage disc. Generally, the rougher the surface finish of the ball bearing, the greater the vibration. Ball bearing axial and radial stiffness ultimately affects motor axial and rocking frequencies. Motor axial and rocking frequencies are the frequencies at which the spindle motor resonates or vibrates. Vibration of the spindle motor creates undesirable acoustics or noise, prolongs the time required for reading and writing information to and from the magnetic disc, and may cause the magnetic head to "crash".
To limit vibration caused by the rough surface finish of ball bearings, grease or other lubrication material is used. However, grease increases friction torque. As a result, greater motor running power is needed to overcome the friction torque. In addition, the grease or lubricating material has an associated outgas or grease vapor. Contaminants carried by the grease vapor become deposited upon the surface of the magnetic disc. These contaminants damage the magnetic head as the magnetic head flies above the surface of the magnetic disc.
In addition, the ball bearings typically rotate within inner and outer races made of steel, usually 52100 chrome steel. The races are typically secured to adjacent components of the disk drive by adhesives. These adhesives have associated vapors which carry and deposit contaminating particles on the surface of the magnetic discs. These particles damage the magnetic head as the magnetic head flies above the surface of the disc.
The magnetic head stores and retrieves information on a magnetic disc. Several types of magnetic heads are currently in use, including metal-in-gap (MIG) heads, thin film (TF) heads and magnetoresistive (MR) heads. Magnetoresistive heads use a magnetoresistive film to read information stored on magnetic discs. The magnetoresistive film is an extremely thin magnetic film which changes in resistance to electrical current as a function of the magnetic flux intercepted by it. For example, the resistance of the film will be high when it is in a strong magnetized region. As the magnetic disc moves past the magnetoresistive head, the polarity of the magnetic field in the region of the magnetoresistive head changes. This represents the information stored on the disc. As a result, the resistance of the magnet, resistive film also changes to affect the current flowing through it. Information on the magnetic disc is decoded by monitoring the current which flows through the magnetoresistive film. Because magnet, resistive heads have a signal independent of speed and are more accurate, data storage density can be increased when magnetoresistive heads are used. Because the magnetoresistive head, and in particular the magnetoresistive film, has current flowing through it, the magnetoresistive head is electrically active. As a result, contact between the magnetoresistive head and the magnetic disc causes an electrical short circuit which may permanently damage the head.
To prevent damage to the magnet, resistive head caused by short circuiting, the magnetic disc must be electrically isolated from ground. In the past, prior disc drives have isolated the magnetic disc from ground by isolation grommets between the spindle motor and the casting or by isolating the whole disc drive casting with rubber feet being attached to the bottom of the casting. However, using grommets or rubber feet for isolation increases manufacturing complexity and increases the space required by the disc drive.
In the process of rotation, considerable static electrical charge is built up on the disc and spindle. This disc potential causes electrical arcs or sparks between the disc and the magnetic head which floats above the disc during rotation of the disc. In inductive read/write magnetic heads, this spark often causes undesirable acoustics or noise and prolongs the time required for reading and writing information to and from the magnetic disc. With magnet, resistive heads, the spark may permanently damage the magnetoresistive head. Consequently, an electrical pathway to eliminate excess static charge and to control disc potential must also be provided. However, the static elimination system, itself, must not generate interfering noise.